No...that's not my argument. I think the logical comparison is to other players at the same age, and I think college and the NBA are comparable if we're focusing solely on demonstrated passing ability.
Sexton averaged 3.7 assists and 2.8 turnovers per 40 at the NBA level at age 19/20.
Curry averaged 3.5 assists and 3.1 turnovers per 40 at the same age in college.
Harden averaged 4.7 assists and 3.8 turnovers per 40 in college when slightly younger than Sexton, 3.2 assists and 2.4 turnovers per 40 in the NBA when slightly older than Sexton.
Kemba averaged 4.8 assists and 2.4 turnovers per 40 in college when slightly older than Sexton
Lillard averaged 4.6 assists and 3.9 turnovers per 40 in college when slightly older than Sexton
Personally, I think AST% is a more valuable metric to cite than per 40 assist numbers when talking about playmaking. To his credit, Sexton doesn't turn the ball over a lot, but that could also be contributed to the fact he simply doesn't pass all that much compared to the amount he has the ball in his hands.
So let's look closer at the names you mentioned...
-Sexton at 19/20 (NBA rookie): 15.3 AST%
Low turnovers, but a very low assist percentage for someone with such a high usage. And I think the "he had bad teammates" excuse doesn't hold water when Dellavedova, Payne and Harrison all had better AST% last year for the Cavs playing with the same teammates.
-Curry at 19/20 (NCAA soph): 21.32 AST%
-Curry as NBA rookie: 24.6 AST%
-Curry career: 31.5 AST%
Total AST% change from age 19/20 to NBA career mark: +10.2%
Total AST% change from NBA rookie year to career mark: +6.9%
-Harden at 19/20 (NCAA soph): 29.2 AST%
-Harden as NBA rookie: 12.3 AST%
-Harden career: 31.8 AST%
Total AST% change from age 19/20 to NBA career mark: +2.6%
Total AST% change from NBA rookie year to career mark: +19.5%
-Kemba at 19/20 (NCAA soph): 29.2 AST%
-Kemba as NBA rookie: 30.2 AST%
-Kemba career: 28.7 AST%
Total AST% change from age 19/20 to NBA career mark: -0.5%
Total AST% change from NBA rookie year to career mark: -1.5%
-Lillard at 19/20 (NCAA soph): 23.3 AST%
-Lillard as NBA rookie: 28.8 AST%
-Lillard career: 29.5 AST%
Total AST% change from age 19/20 to NBA career mark: +6.2%
Total AST% change from NBA rookie year to career mark: +0.7%
So outside of Curry, none of the other three guys you listed made big gains from an AST% perspective from what they showed at 19/20 year old.
Harden is the only one to make a massive gain from his rookie NBA season. In all fairness there, Harden did have some major early career struggles as a playmaker, but given his college numbers I think his rough start in the NBA can be directly attributed to the fact he was playing with two 32%+ usage guys while be relegated to an off ball 6th man role in OKC. The moment he got to Houston and got back into a primary offensive role his playmaking immediately spiked back up to what he showed at Arizona State.
Sexton was pretty significantly worse than all of those guys at the same age (granted it was against better competition), so it's hard to be optimistic he'll see big gains as a playmaker. Especially now that the Cavs have added Garland who will presumably be on the ball a lot, perhaps even more than Sexton.
It wouldn't be completely unprecedented for Sexton to make a huge leap, but very few guys do it. I went back and looked at everyone this decade and the only guys I was able to find to make 15 AST% leap were...
D'Angelo Russell went from 21.2 AST% in Year 1 to 41.3% in Year 4.
Devin Booker went from 15.8 AST% in Year 1 to 34.1% in Year 4.